1) Field of the Invention
The present invention relates to aircraft wings and, more particularly, to a lift augmentation system for increasing lift of a multi-element aircraft wing by controlling boundary layer flow over the aircraft wing.
2) Description of Related Art
Takeoff and landing performance are two principal design objectives for transport aircraft. Any aircraft design is limited to a maximum takeoff weight which is related to the runway length. For a given runway length, higher lift levels permits the maximum take-off weight to be increased. Equivalently, for a given weight, higher lift allows for lower stall speed and shorter runway length. From an operational perspective, high-lift capability results in access to a larger number of airports. Whether the requirement is for a larger payload or for shorter runways, superior high-lift capability is a key objective of the aircraft manufacturers.
For a given aircraft weight, it is possible to reduce stall speed by either increasing wing area or increasing the maximum lift coefficient (CLmax). Increasing the wing area is undesirable since it results in higher cruise drag. Therefore, increasing CLmax is more desirable.
Efficient high-lift systems provide crucial performance advantages for both military and commercial aircraft. In the context of military aircraft, the ability to land in remote and austere fields is required such that military transports with short runway capability can effectively increase the global reach of the military force. With respect to commercial transports, the economical impact of high-lift systems is substantial. For example, an increase in the CLmax results in an increased payload capacity for fixed approach speed, an increase in take-off L/D results in an increase in payload or increased range, and an increase in the lift coefficient at a constant angle of attack reduces the approach attitude and results in shortened landing gear, i.e., reduced aircraft weight.
Another aspect of the economic advantage attributable to enhanced high-lift capability relates to environmental regulations. A growing number of communities enforce stringent noise limits in airport environments, resulting in limited hours of operation of the aircraft. Moreover, aircraft that do not operate within permissible noise limits are financially penalized or even prohibited from operating in and out of certain airports. For example, to comply with environmental regulations, some aircraft have been forced to reduce payload, as well as reduce take-off and lift-off speeds during the initial climb. However, with fewer passengers on board, operating the aircraft was no longer economically viable. Consequently, there is a great economic incentive to develop aircraft with improved takeoff and landing performance.
The aerodynamic design is especially challenging for take-off and landing conditions where the fluid flow is dominated by viscous effects. Techniques for altering the viscous flow structures at these high-lift conditions are highly desirable due to the increased potential for improved efficiency. Over the years, a variety of flow control strategies have been developed for a wide range of aerodynamic applications, such as various active and passive systems, actuators, and mechanisms for altering the flow over a wing or delaying boundary layer separation. For example, U.S. Pat. No. 6,905,092 to Somers discloses a laminar-flow airfoil that includes fore and aft airfoil elements and a slot region located therebetween. The fore and aft airfoil elements induce laminar flow over substantially all of the fore airfoil element and laminar flow in the slot region.
Current aircraft achieve high levels of lift by employing systems that are deployed only during take-off and landing. These systems usually consist of a movable leading-edge slat and one or more trailing-edge flaps. When deployed, the wing transforms into a multi-element configuration, effectively increasing camber and chord length and resulting in added lift. The flow over a multi-element high-lift system is highly interactive. For instance, the trailing-edge flap is strongly influenced by the downwash generated by the lift on the main wing.
The maximum lift that can be achieved by such a multi-element system is limited by viscous effects resulting from strong adverse pressure gradients. The maximum lift level achieved can be limited by boundary layer separation in the vicinity of the slat and main wing leading edge, as well as by boundary-layer thickening or separation on the trailing edge of the main wing or on the flap(s). Lift can also be limited by boundary-layer thickening or separation on the trailing edge of the main wing or on the flap(s). In addition, the maximum lift level can be limited by the bursting of the viscous wake from the slat or main wing as it passes through the high pressure gradients developed by the flap. In this case, the boundary layers on each of the high-lift components may be attached, but the rapid spreading of the viscous wakes limits the maximum lift that can be achieved.
It would therefore be advantageous to provide a system that is capable of controlling boundary layer flow over a multi-element aircraft wing for improved aerodynamic performance of a multi-element wing. Moreover, it would be advantageous to provide a system that is adaptable to a wide range of angles of attack and flight conditions.